Spinal interbody fusion is a commonly performed procedure for degenerative disorders. A spinal fixation device is usually used to control intervertebral movement, and at present the spinal fixation devices generally use bone screws as the basic elements and the screws are attached to a rod or a plate outside of the spinal column to constitute a complete fixation system. The screw based fixation systems can be implanted to the spinal column from posterior, anterior or lateral side and they have been proved in biomechanical tests to be capable of controlling six-way of intervertebral movement, including axial rotation, lateral bending, and flexion and extension. Take posterior inserted bone-screw-based spinal fixation system for instance, the implantation method includes dissecting back muscles until posterior bony elements of the spinal column are exposed. Four screws are then threaded through the pedicles into two adjacent vertebral bodies, and connected two plates or rods on both sides.
Since early 1990, interbody fusion cage has been popularized until today and are used to maintain intervertebral height and to enhance fusion. The cage can be inserted from posterior, posterolateral, anterior or lateral trajectory. Bone graft is put inside the cage and fusion will occur within an expected time frame. The so-called stand-alone cage uses a fusion cage without any supplemented posterior or anterior fixation device. The biomechanical weakness of the stand-alone cage is that the cage can control flexion and lateral bending movement, but effectiveness in controlling extension and axial rotation movement is not as good as expected. Therefore, intervertebral movement is not fully under controlled by using the stand alone cage, and the fusion rate is reported suboptimal. To obtain stability which is a prerequisite for successful fusion, a supplemented screw based spinal fixation device is recommended.
Based on the understanding of this screw based spinal fixation device, US patent application No. 2004/0034353A1 filed on Aug. 11, 2003 discusses an apparatus and method for anterior spinal stabilization. This application requires a pre-installed rivet or screw to undergo the entire implantation process, which is complicated and too time consuming. This process is not applicable for patient suffered from Osteoporosis for the implantation process destroys the vertebral body too much.
Another US patent application No. 2005/0021029A1, filed on Jul. 25, 2003 discusses an annulus repair system, instruments and techniques. This application is aimed for damaged intervertebral disc such that almost the materials involved in the process is related to non-rigid material, which is not able to withstand the twist, bend and rotation of the spine.
Still another US patent application No. 2004/0220670A1 filed on Feb. 6, 2004 involved an articular disc prosthesis and method for treating spondylolisthesis. In this application, there is a raised plate-like segment aimed for sliding movement of the intervertebral body after the insert is implanted. This structure does have the function to prevent intervertebral movement in sideways and twist. The movement prevention function still cannot prevent bending movement of the intervertebral body. Another US patent application No. 2005/0159813A1 filed on Jan. 15, 2004 involves a spinal implant construct and method for implantation. It is not applicable for patient suffered from Osteoporosis for the implantation process destroys the vertebral body too much.
U.S. Pat. No. 5,591,235 filed on Mar. 15, 1995 involves an annular and hollow column extending all the way through the intervertebral body, which destructs too much of the intervertebral body. A rod intended to be fitted into the slot in the hollow column is narrow and provides no space for new bone growth. In addition, the edge of the rod is blunt and it cannot extend into the intervertebral body directly. A previously defined slot has to be ready before the use of the rod, which is too complex and complicated. U.S. Pat. No. 6,558,424, filed on Jun. 28, 2001 provides a modular anatomic fusion device involves a wedged block for insertion into the intervertebral space and the rough surface formed on the top and bottom faces of the wedged block is to increase friction with the vertebral body. With the help of the rough surface, the positioning of the modular anatomic fusion device is fixed relative to the intervertebral body. However, it still cannot withstand the twist and bending movement of the intervertebral body. In yet another US patent application No. 2003/0083746A1, a porous annular column is provided to replace the intervertebral disc. It is quite obvious that after the porous annular column is inserted between the intervertebral bodies, the spacer cannot withstand the bending, twisting or rotation of the spine.
In summary, to achieve a successful interbody fusion, screw based or non-screw based spinal fixation devices are used to control six-way of the intervertebral movement, in conjunction with a fusion cage. These fixation devices require complicated assembling procedure and may be hazardous to the human body.